Cumulative Update 3 for Exchange Server 2013 resolves issues that were found in Exchange Server 2013 CU1/CU2 since the software was released. This update rollup is highly recommended for all Exchange Server 2013 customers.

The CU3 package can be used to perform a new installation, or to upgrade an existing Exchange Server 2013 installation to CU3. You do not need to install Cumulative Update 1 or CU2 for Exchange Server 2013 when you are installing CU2.

This Cumulative Update includes Schema and AD changes, so make sure you run PrepareSchema / PrepareAD. After updating, the schema version will be 15283. For list of schema versions see wiki

Cumulative Update 2 for Exchange Server 2013 contains the following major changes

Usability improvements when adding members to new and existing groups in the Exchange Administration Console

Online RMS available for use by non-cloud based Exchange deployments

Improved admin audit log experience

Windows 8.1 / Internet Explorer 11 no longer requires using OWA Light

Cumulative Update 3 for Microsoft Exchange Server 2013 contains the fix for the security issue that is described in Security Bulletin MS13-061 and resolves the issues that are described in the following Microsoft Knowledge Base (KB) articles:

Please Note: Cumulative Updates can be installed directly, i.e. no need to install RTM prior to Cumulative Updates, also note that once installed, you can’t uninstall a Cumulative Update nor any of the installed Exchange server roles. The order of upgrading servers is irrelevant, unlike with previous generations of Exchange.